Refer back to your previous question. This is from the Missouri Botanical garden:Plants are short-lived, and dead patches often begin to appear after several years. In St. Louis, foliage usually declines as the summer progresses. In other words in winter you will see dead plants as is the case with most flowering perennials.

Here is a link that might be useful: Propagating Perennials

how about periwinkle?

Periwinkle can be invasive but why not you have a lot of area to cover, but keep track of it. Use staggered rows so it fills in quickly. Just make certain you amend the soil before planting. Euynomous would do as well, like Emerald Gaity, multi-colored leaves.

Here is a link that might be useful: Propagating Perennials

Did you plant Snow-in-Summer? Did you find it invasive?

Is it a sunny, relatively dry area? If so, prairie winecup, Callihroe involucrata, might be a worthwhile choice. It blooms from late spring till frost. Snow in summer turns yellow and dies out in the middle, so it looks terrible after a while.